1. Field of the Invention
This invention relates generally to thermoelectric materials and more particularly to superlattice structures used in thermoelectric materials and techniques for providing superlattice structures having enhanced thermoelectric figures of merit.
2. Description of the Related Art
As is known in the art, a thermoelectric material refers to a material capable of directly converting thermal energy into electrical energy and vice versa or capable of cooling a material when a current is flowing in the desired direction. Such materials include, for example, heavily doped semiconductor materials. In a thermoelectric generator, for example, the Seebeck voltage generated under a temperature difference drives a current through a load circuit. Typical thermoelectric generators employ a radioisotope, a nuclear reactor or a hydrocarbon burner as the heat source. Such generators are custom made for space missions, for example. Some materials such as tellurides and selenides are used for power generation up to a temperature of about 600xc2x0 centigrade (C). Silicon germanium alloys provide better thermoelectric performance above 600xc2x0 C. and up to about 1000xc2x0 C. With presently available materials, conversion efficiencies in the five to ten percent range are typically expected.
It would, however, be desirable to provide such thermoelectric materials having higher conversion efficiencies. Such devices may then be effectively employed in an apparatus such as an automobile to thus increase the fuel efficiency of the automobile.
Superlattice structures, in general, are known and typically comprise a composite made of alternating ultrathin layers of different materials. Typically, the superlattice has an energy band structure which is different from, but related to, the energy band structure of the component materials. By the appropriate choice of materials (and other factors discussed below), a superlattice having a desired energy band structure and other characteristics can be produced. Superlattices have many uses, including, but not limited to, use in the field of thermoelectric power generation or cooling.
The fabrication of a superlattice by molecular beam epitaxy (MBE), or other known epitaxial growth techniques, is generally known. However, the choice of materials and the relative amounts of the materials which make up the superlattice are predominant factors in determining the characteristics of the superlattice. For use as a thermoelectric material in power generation applications, it is desirable to choose the materials, and their relative amounts, so that the thermoelectric figure of merit, ZT, is maximized.
The dimensionless thermoelectric figure of merit (ZT) is a measure of the effectiveness of the material for both cooling and power conversion applications and is related to material properties by the following equation:
ZT=S2"sgr"T/K,
where S, "sgr", K, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and temperature, respectively. The Seebeck coefficient (S) is a measure of how readily electrons (or holes) can convert thermal to electrical energy in a temperature gradient as the electrons move across a thermoelement. The highest useful Seebeck coefficients are found in semiconductor materials with a high density of states at the Fermi level. In theory, to maximize the thermoelectric figure of merit ZT, one would try to increase or maximize the Seebeck coefficient S, electrical conductivity "sgr" and temperature T and minimize the thermal conductivity K. However, in practice, this is not so simple. For example, as a material is doped to increase its electrical conductivity ("sgr"), bandfilling tends to lower the Seebeck coefficient S and the electronic contribution, Ke, to the thermal conductivity K increases. At a given temperature, the thermoelectric figure of merit ZT for a given material is maximized at an optimum doping level. In most materials, the thermoelectric figure of merit ZT is maximized at doping levels of approximately 1019 cmxe2x88x923. Since increasing (or decreasing) one parameter may adversely decrease (or increase) another parameter, it is generally difficult to achieve higher values for ZT. It should of course be appreciated that increasing "sgr" increases Ke, but decreases S and vice-versa. Currently, the best thermoelectric materials have a maximum ZT of approximately 1. The ZT values are below one at temperatures both below and above the temperature at which they achieve the maximum value.
The thermoelectric figure of merit ZT in conventional (bulk) thermoelectric materials is also related to the thermoelectric materials factor (b*) which may be expressed as:
b*=xcexcm*3/2/KL
in which:
xcexc is the carrier mobility;
m* is the density of states effective mass; and
KL is the lattice thermal conductivity.
The precise relationship between the thermoelectric materials factor b* and the thermoelectric figure of merit ZT is relatively complex.
A superlattice provides the opportunity to enhance the values of ZT for a number of reasons. Under appropriate conditions, the Seebeck coefficient of a superlattice increases as the thickness of a period of a quasi-two-dimensional superlattice decreases. The carrier mobility is generally increased by means of modulation doping and xcex4-doping, and this effect has previously been demonstrated in Si/SiGe strained-layer superlattices. Furthermore, the lattice thermal conductivity of a small-period superlattice is expected to be substantially lower than the average of those for the component materials because of augmented phonon-interface scattering effects.
In view of the above, it would be desirable to provide a superlattice structure which has a thermoelectric figure of merit which is higher than that heretofore achieved.
In accordance with the present invention, a superlattice structure includes a plurality of alternating layers of at least two different semiconductor materials. First ones of the layers correspond to barrier layers and second ones of the layers correspond to well layers. It should be understood that there are cases where xe2x80x9cwellxe2x80x9d layers act as both xe2x80x9cwellxe2x80x9d and xe2x80x9cbarrierxe2x80x9d layers and xe2x80x9cbarrierxe2x80x9d layers act as both xe2x80x9cwellxe2x80x9d and xe2x80x9cbarrierxe2x80x9d layers. For example, in GaAs/AlAs superlattices, GaAs (AlAs) layers are xe2x80x9cwellxe2x80x9d (xe2x80x9cbarrierxe2x80x9d) layers for the xcex93- and L-valleys, whereas they are xe2x80x9cbarrierxe2x80x9d (xe2x80x9cwellxe2x80x9d) layers for the X-valleys. Each of the well layers are provided having quantum well states formed from carrier pockets at various high symmetry points in the Brillouin zone of the superlattice structure. With this particular arrangement, a superlattice having a relatively high thermoelectric figure of merit is provided.
The superlattice structures of the present invention may be used for various purposes including, but without limitation, thermoelectric power generation, cooling, and electronic devices. In one embodiment, the barrier layers of the thermoelectric material are provided from one of AlGaAs and AlAs and the well layers are provided from one of GaAs and AlGaAs. In another embodiment, the alternating layers are provided one of silicon (Si) and silicon-germanium (SiGe), and one of silicon-germanium (SiGe) and Ge (germanium).
As mentioned above, the same layer (for example GaAs layer in a GaAs/AlAs superlattice) can be used as both a xe2x80x9cbarrierxe2x80x9d and a xe2x80x9cwellxe2x80x9d layer in the same superlattice structure. This means that quantum wells for the xcex93- and L-valleys in a GaAs/AlAs superlattice (or xcex94-valleys in a Si/Ge superlattice) are formed within the GaAs (Si) layers, whereas quantum wells for the X-valleys (or L-valleys in a Si/Ge superlattice) are formed within the AlAs (Ge) layers of the superlattice, in the same superlattice structure. Therefore, using the carrier pocket engineering technique, both GaAs and AlAs layers are used as both the barrier and the well layers so that the thermoelectric coefficients of the GaAs/AlAs superlattice are enhanced after the application of the carrier pocket engineering technique. The same thing is true, of course, for the Si/Ge superlattice.
For example, a superlattice according to the present invention may be made of alternating layers of a first material and a second material, where the first material comprises silicon (Si) and where the second material comprises SiGe or both materials could be SiGe with different Si to Ge stoichiometric ratios.
It has been discovered that the figure of merit ZT for selected quantum well superlattice structures may be enhanced by the process of carrier pocket engineering in which the structures and geometries of a given superlattice system are optimized in such a way that the optimized structure maximizes the value of ZT for the whole superlattice.